MEMORANDUM 

TO: Docket ID No. EPA-HQ-OAR-2015-0764 

FROM: Alexis McKittrick and Mark de Figueiredo, EPA/CCD 

DATE: January 21, 2016 

SUBJECT: Duration of Leaks in Time Variable of Equation W-30 of Subpart W 

The U.S. Environmental Protection Agency (EPA) is proposing revisions to 40 CFR part 

98, subpart W (hereafter referred to as “subpart W”) in order to align subpart W with proposed 

new source performance standards (NSPS) for the oil and gas source category (40 CFR part 60, 

subpart OOOOa; hereafter referred to as “NSPS subpart OOOOa”) (see 77 FR 49542; September 

18, 2015). Specifically, the EPA is proposing to add the optical gas imaging (OGI) monitoring 

method proposed in NSPS subpart OOOOa to subpart W. In addition, the EPA is proposing to 

add new leaker emission factors in subpart W for industry segments and/or components that 

currently do not have leaker emission factors so that the new monitoring data can be used when 

estimating greenhouse gas emissions in subpart W. If finalized, proposed NSPS subpart OOOOa 

would require facilities also reporting under subpart W to perform fugitive emission monitoring 

separate from the requirements currently in subpart W and repair leaking equipment. Facilities 

that would be newly subject to proposed NSPS subpart OOOOa would be required to do an 

initial monitoring survey, conduct a second monitoring survey one month later, then begin 

conducting semi-annual surveys. 

Equation W-30 of subpart W is used to calculate equipment leak emissions using leak 

detection survey data, leaker emission factors, and a time variable, Tp,z, to account for the time 

an equipment component was leaking. This time variable is currently defined at 40 CFR 

98.233(q) as follows: 

The total time the surveyed component “z”, component type “p”, was assumed to 

be leaking and operational, in hours. If one leak detection survey is conducted in 

the calendar year, assume the component was leaking for the entire calendar year, 

accounting for time the component was not operational (i.e., not operating under 

pressure) using engineering estimate based on best available data. If multiple leak 

detection surveys are conducted in the calendar year, assume that the component 

found to be leaking has been leaking since the previous survey (if not found 

leaking in the previous survey) or the beginning of the calendar year (if it was 

found leaking in the previous survey), accounting for time the component was not 

operational using engineering estimate based on best available data. For the last 

leak detection survey in the calendar year, assume that all leaking components 
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continue to leak until the end of the calendar year, accounting for time the 

component was not operational using engineering estimate based on best available 

data. 

The current language for the definition of the time variable specifically considers a first 

leak survey and a last leak survey in the year, but does not provide specific language with respect 

to the duration of any “intermediate” survey conducted between the first and last survey. 

Therefore, the EPA is proposing to amend the definition of the time variable, Tp,z, of Equation 

W-30 to clarify what reporters should do if more than two leak surveys are conducted in a year. 

In addition, in response to comments as part of the 2010 subpart W final rule, the EPA 

further clarified how the time variable is intended to be applied: 

...EPA recognizes the commenter’s concern that assuming a leak duration of 365 

days may overestimate emissions. Conversely, there will be leaks that start after a 

leak survey is conducted and therefore an underestimation may occur as well. 

Regardless, in today’s final rule, EPA allows reporters the option to perform 

subsequent facility-wide leak detection surveys and to adjust their emissions to 

account for components that are subsequently found to be leaking or not leaking, 

respectively. EPA emphasizes that adjustment of emissions is not allowed based 

on repair records alone. Reporters must assume that a leaking component has 

been leaking starting from the beginning of the calendar year. In addition, if only 

one leak detection survey is conducted during the calendar year, the reporter must 

assume that the duration of the leak is 365 days unless the leak is fixed and a 

subsequent official leak detection survey is conducted for an entire facility 

proving that the leak and others were repaired. A goal with leak detection is to get 

a facility wide snapshot of equipment leak emissions. If a reporter finds certain 

components leaking during a specific survey they are usually not addressed 

immediately. During the time leaks are being addressed, other leaks will appear 

which only subsequent facility wide leak detection surveys will ascertain.1

In other words, subpart W specifies that any leak that is identified during a leak detection 

survey is assumed to continue to leak until the next complete survey indicating the leak has been 

repaired (or until the end of the reporting year, if identified during the last survey conducted in a 

reporting year). 

 

1

 U.S. Environmental Protection Agency. Mandatory Greenhouse Gas Reporting Rule Subpart W – Petroleum and 

Natural Gas: EPA’s Response to Public Comments. November 2010. Docket Item No. EPA-HQ-OAR-2009-0923-

3608. Response to Comment Number EPA-HQ-OAR-2009-0923-1014-9, pp. 1281–1282. 
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As the proposed NSPS subpart OOOOa requires repair of fugitive leaks, the EPA 

considered the need to further amend the definition of the time variable, Tp,z, of Equation W-30 

beyond the proposed clarification discussed above to account for these repairs. 

Option 1 (Current Requirement). As stated above, currently in subpart W, any leak that 

is identified during a leak detection survey is assumed to have been leaking since the previous 

complete leak survey or the beginning of the reporting year, whichever is more recent, and the 

leak is assumed to continue to leak until the next complete survey indicating the leak has been 

repaired (or until the end of the reporting year, if identified during the last survey conducted in a 

reporting year). 

Option 2. Consider each complete leak detection survey independently when multiple 

complete surveys are conducted in a reporting year. For the last complete survey of the reporting 

year, the leak duration is from the previous leak detection survey until the end of the reporting 

year, just as currently required. However, for the first complete survey in a reporting year, the 

leak duration is from the start of the reporting year until the date of the first leak survey (instead 

of being considered to be leaking up to the next leak survey). The leak duration for an 

intermediate leak detection survey is the time from the date of the previous leak detection survey 

through the date of that intermediate leak detection survey (instead of being considered to be 

leaking up to the next leak survey). 

Option 3. Allow remonitoring of individual components to count as a leak survey for that 

component for the purposes of the time period considered in Tp,z. Assume the leak duration 

begins on January 1 of the reporting year or on the date of the previous non-leaking survey 

(whichever is later) and continues until the next survey of that component or until the end of the 

reporting year (whichever is earlier). In this option, the component is assumed to leak until 

December 31 only when the component is not repaired. Otherwise, the leak period ends with the 

remonitoring of the repaired component. 

Option 4. Allow remonitoring of individual components to count as a survey, as in 

Option 3, but do not confine the duration of a leak to a reporting year. For example, if a leak is 

not repaired in reporting year (RY) 2016, than that leak is assumed to continue into RY2017. 

Similarly, if the first survey conducted in RY2017 identifies a leak, then that leak is assumed to 

occur from the last monitoring event in RY2016. The reporter would then need to attribute the 

duration of each leak attributable to a given reporting year (for leaks spanning reporting years) in 

order to determine the value for Tp,z of Equation W-30 for that reporting year. 

In order to provide consistency and clarity for subpart W reporters in the proposed rule 

amendments, the EPA chose Option 1, which retains the existing requirements for application of 

time variable, Tp,z, of Equation W-30.